Pneumonia is a common disease and a frequent cause of morbidity and mortality. S. pneumoniae is the most common cause of community-acquired pneumonia and a major health concern. Understanding lung host defense in response to this organism and successful resolution of inflammatory and immune responses is important and likely to have impact on novel ways to intervene therapeutically. Lung macrophages are a critical arm of host defense in healthy lungs and in response to infection or any injury. The classification of subpopulations is very topical and controversial. We have focused on macrophage categories designated by surface markers that identify three subpopulations; alveolar macrophages (AMs), interstitial macrophages (IMs) and inflammatory macrophages (InfMs). In unchallenged lungs, our preliminary studies show that AMs are the only lavageable macrophage present in the airways/alveoli. Digests of PBS-treated (control) lungs reveal that of total lung macrophages, about 28% are AMs, 50% are IMs, and 22% are InfMs. Gene profiling of isolated subpopulations reveals very distinct patterns of mRNA expression by each subpopulation, suggesting in health, each subpopulation has unique functions. During pneumonia, the percentage and number of each subpopulation changes dramatically. Initially, the total number of lung macrophages increases 3-4-fold, which reflects a decrease in the number of AMs, an increase in InfMs and no change in IMs. By 14 days, the numbers are nearly back to those in healthy lungs, but AMs are now from both bone marrow and lung-resident origins. Thus, these subpopulations are very dynamic and very likely to have specific functions that may change within each subpopulation over the course of a pneumonia. Furthermore, these cell-specific changes in number and function are very likely to be modulated by the alveolar microenvironment, which changes during infection and the immune response. Our studies use lung-protected radiation and bone marrow reconstitution that generates chimeric mice but does not alter the lung macrophage subpopulations. The Aims test the overall hypothesis that subpopulations of lung macrophages each play specific and important roles in host defense during bacterial pneumonia, with the expectation of identifying new and important mechanisms underlying these processes. Aim 1 determines trafficking kinetics, including changes in the subpopulations of lung macrophages during pneumonia and the mechanisms through which these changes occur. Aim 2 determines the function of each macrophage subpopulation. Aim 3 determines the effect of the alveolar microenvironment on the kinetics and function of macrophage subpopulations. These studies test the hypothesis that trafficking and function of particular subpopulations will be modulated by parenchymal ICAM-1, by the CX3CL1/R1 (fractalkine) axis, and by Nrf2-mediated cytoprotection against oxidant damage.